According to some authorities (Williams, for example), the relative pronouns
that and which may be used interchangeably.

example: The lion which rested by the river seemed well fed.

The lion that rested by the river seemed well fed.

According to other authorities (Lanham, for example), that and which
have useful differences and should not be used interchangeably. They argue
that which should introduce a parenthetical clause (= a nonessential
or nonrestrictive clause, also called a 'free element') and that that
should introduce an essential clause (= a restrictive clause).

example a: The
lion, which rested by the river, seemed well fed.

example b:
The lion that rested by the river seemed well fed.

In example a,
commas set off the adjective clause because the sentence concerns only one
lion and the thought remains complete even if the adjective clause is removed;
the adjective clause is not essential, it is a free element and can be removed
without changing the sentence meaning. In example b, no commas are
used because the sentence concerns several lions, one of which is by the river.
Here the relative pronoun that tells us to focus on a subset of the
whole: that restricts the meaning of lion and is essential for understanding
which lion.

Because the use of that and which as restrictive and nonrestrictive
elements conveys useful information to the reader, I support the distinction.